I was recently researching a similar question AC (Canada to US) connecting w UA (US to Asia). AC says $75/first luggage, but UA shows $0/first luggage.

Found the following script from AC's website under:
"If your itinerary includes travel with one of our codeshare partners or with another airline, it’s important to know that the baggage policies of the other carrier may apply"

For Canada & US, but its beginning AND ending
For itineraries beginning and ending in Canada or the U.S., the selecting carrier1 will decide which baggage rule to apply to the entire itinerary.
1. The selecting carrier is the carrier whose designator code is identified on the first flight segment of your interline ticket.

Travel outside of Canada & US
For all other itineraries: As per IATA Resolution 302 (Most Significant Carrier rule), if the most significant carrier of your journey is Air Canada, Air Canada rouge or Air Canada Express, the baggage rules and charges of Air Canada will apply. If the most significant carrier is another airline, different baggage rules and charges may apply.

Gonna be interesting to see how this will pan out.